Free space optics (FSO) is a telecommunication technology that uses optical beam in free space to transmit data between two points. The data can be any user information that is to be transmitted from a transmitter to a receiver within the communication system.
The free space optical communication system provides a number of advantages over the other communication system such as wireless communication system. For instance it provides higher rate of data transfer and chances of eavesdropping or interception of data is less. Further, FSO does not require licensing and distribution of bandwidth. Additionally, such a communication can be useful where the physical connection between the transmitting station and the receiving station is difficult. For example, in cities, the laying of fiber optic cables can be expensive and, in some instances, it can be much more complex than setting up an FSO.
However, FSO suffers from a number of major drawbacks. Some such drawbacks are the effects of atmospheric turbulence or distortions, aberrations or other perturbations, subjected to an optical wave and the absorption of the laser (light) beam by the intervening medium. Such random and uncontrolled aberrations in the optical wavefront are often undesirable as they degrade the performance of the optical system. Further, these drawbacks lead to variations in the beam wavefront and intensity which results in inaccuracies in the receiving station.
In order to increase the data content in the beam, orbital angular momentum (OAM) states of a laser beam may be used. However, laser beam carrying multiple OAM states gets severely affected by the presence of aberrations in the medium through which it travels.
Therefore, in order to overcome the drawbacks, there have been many important developments in the area of free space optical communication. Attempts have been made to come up with a robust system with a very large information carrying capability.
Reference is made to a non-patent literature, Gibson, Graham, et al. Optics Express 12.22 (2004): 5448-5456, wherein a free space communication system is proposed, where light beams carrying orbital angular momentum states is used to carry the information content. The receiving station uses a multiplex hologram to detect the presence or absence of various OAM states. However, the major drawback of this communication system is the susceptibility of OAM modes to aberrations introduced by the intervening medium.
Reference is made to an application, U.S. Pat. No. 7,343,099, which discloses an FSO where the intensity fluctuation in the laser beam is minimized by employing laser speckle tracking and locking principle. However the scheme involve certain number of operations at each instant of the laser beam, making the operating speed of the FSO limited by the capability of the other components involved in the tracking and locking mechanism.
Reference is made to a non-patent literature, Feng, Ian H. White, and Timothy D. Wilkinson, Journal of Lightwave Technology 31.12 (2013): 2001-2007, which proposes a free space communication system using a two-electrode tapered laser followed by a liquid-crystal SLM. The system allows large data content with the SLM facilitating adaptive aberration correction due to the atmosphere. However the rate of aberration correction by the SLM is limited by the response time of the liquid crystal molecules.
Reference is made to a non-patent literature, Hao Huang, et al., Opt. Lett. 39, 197-200 (2014), which discloses a 100 Tbit/s free-space data link system which is made possible by combining orbital angular momentum, polarization, and wavelength based modulation of the laser beam. Although the proposed system has a very large data transfer rate, yet the system suffers from the same limitations due to aberrations as by any other OAM state based FSO.
Reference is made to a non-patent literature, Liu, Wei, et al., Optics and Laser Technology 60 (2014): 116-123, wherein a holographic modal wavefront sensor is disclosed to correct the laser beam in an FSO from aberrations thereby improving the fiber coupling efficiency. However this literature only recited about the mechanism to correct the beam from aberrations and there is no attempt to modify the beam modulation scheme. Further the modal wavefront sensor used charge-coupled device (CCD) detector and thus its speed was limited to the frame rate of the CCD.
Reference is made to patent application, CN 102288305 B, wherein a wave-front sensor of a self-adaptive optical system and a detecting method thereof, is disclosed. The invention provides an adaptive optics wavefront sensor, consisting a binary light intensity modulator, a focusing lens, a single-mode optical fiber, a non-array photodetector and computer components, based on the binary orthogonal aberration mode filtering and detection principle. However this prior art discloses the use of orthogonal aberration modes such as Zernike modes to detect the aberrations present in the incident beam and it does not describe use of the presence or absence of such orthogonal aberration modes as means to encode user information. Besides, the above prior art has made use of binary light patterns which are binarized version of the Zernike mode itself and no tilt was added to the Zernike mode. Further, the light intensity at one location is only considered for each Zernike mode.
Reference is made to patent application, U.S. Pat. No. 5,120,128 A, wherein a wavefront aberration sensor includes a beam splitter, one or more aberration sensor modules and a photodetector for sensing total light power. The aberration sensor modules each provide two voltage outputs from a pair of photodetectors. Differences in the voltage pairs are normalized by the total light power to represent signed aberration amplitudes of phase aberrations present in an input optical beam. The aberration amplitudes may be combined in a digital computer to provide a reconstructed wavefront. However, this prior art discloses a method to measure the presence or absence of a given aberration by dividing the incident beam into two beam and by adding and subtracting equal amount of aberrations from the two beams but it does not make use of binary hologram to realize two copies of a single beam with positive and negative amount of aberrations in the two diffracted beams as is done in the present invention. Further, the above prior art does not teaches about the presence or absence of orthogonal aberration modes as means to encode user information.
Thus, in view of the existing free space optical communication system as discussed above, it is ascertained that there has been a steady progress as far as the capability of the communication system is concerned. However, there exist a need for an FSO communication system that is inherently less susceptible to disturbances such as wavefront distortions and intensity fluctuations introduced by the atmosphere and caters better and faster communication of data in free space, even in presence of distortions introduced by the atmosphere.